In optical fiber manufacturing, during the drawing process, a coating resin is coated around the circumference of a silica glass fiber immediately after the silica glass fiber is drawn to prevent strength reduction of the resulting optical fiber. An ultraviolet-curable resin, particularly a urethane-acrylate-type or an epoxy-acrylate-type, is generally used as the coating resin for the optical fiber.
An optical fiber increases transmission loss due to various external stresses and microbends caused by such external stresses. To protect the optical fiber from such external stresses, the optical fiber is usually coated with two layers (a soft layer and a hard layer) of coatings. For the inner layer, which directly contacts the silica glass, a soft resin with low Young's modulus is used as a buffer layer (hereafter a primary layer); and for the outer layer, a hard resin with high Young's modulus is used as a protective layer (hereafter a secondary layer). Conventionally, a resin with less than 3 MPa in Young's modulus is used as the primary layer, and a resin with more than 500 MPa in Young's modulus is used as the secondary layer. Also, when it is needed, a very thin color layer is added for identification purpose.
Hereafter, in this specification, a silica-glass fiber coated with a primary layer and a secondary layer is called an optical fiber; and the combination of the primary layer and the secondary layer is called a first coating layer.
Because the optical fiber itself may not have desired strength, in some applications, the optical fiber is further coated with a second coating layer such as a thermoplastic resin or an ultraviolet-curable resin, and used as a tight-buffered optical fiber. (for example, see Japanese Patent No. 3,955,829). Furthermore, by adding a sheath, it can be used as an optical fiber cable.
When such an optical fiber cable is exposed to a humid and hot environment for a long period of time, transmission loss may increase. To create highly reliable optical fibers, which prevent the increase in transmission loss even if they are exposed to a humid and hot environment for a long period of time, various suggestions such as using a metal film or a waterproof adulterant around a tight-buffered optical fiber have been made (for example, see Japanese Patent Application Laid-open No. 2004-85741).
Because the popularity of optical fibers has grown in recent years, the number of optical fiber cable applications has similarly grown, which indicates that the environments where optical fiber cables are used have diversified and new cable structures have been developed. Because of that, the long-term reliability required for optical fiber cables has become stricter.
With the situation as stated above, an optical fiber cable, which is less likely to increase transmission loss when it is exposed to a humid and hot environment, is being considered.
However, most of the approaches disclosed until today have focused on ways to prevent moisture from reaching a tight-buffered optical fiber, or on ways to reduce the amount of the moisture reaching the tight-buffered optical fiber by changing the cable structure. In those cases, other harmful effects or reduction in manufacturability are occurred. For example, if the metal film or the waterproof adulterant is used around a tight-buffered optical fiber as described in Japanese Patent Application Laid-open No. 2004-85741, when the tight-buffered optical fiber is removed from an optical fiber cable, the metal film or the waterproof adulterant interferes with the removal process and reduces its manufacturability.